VECSELs are one of the most promising high brightness laser sources and offer many advantages compared to edge emitters, like addressable 2D-array arrangements and circular beam shapes. In standard setups the external cavity is composed of macroscopic optical elements, which are very bulky and need sophisticated adjustment. By realising the external optical components on a wafer and bonding this wafer to the GaAs wafer carrying the semiconductor components of the VECSEL, it is possible to manufacture many thousands of micro-VECSELs in parallel and test them directly on the wafer like VCSELs (vertical cavity surface-emitting lasers). G. A. Keeler et al. “Single Transverse Mode Operation of Electrically Pumped Vertical-External-Cavity Surface-Emitting Lasers with Micromirrors”, IEEE Photonics Technology Letters, Vol. 17, No. 3, March 2005, pp 522-524, describe the fabrication of micromirrors on a glass substrate using a micromolding process, which substrate is then bonded to the GaAs wafer with an index-matched optical epoxy. Such micro-VECSELs show good performance (M2 close to 1).
The external cavity of the VECSEL is defined by a spherical mirror on an optically transparent substrate bonded on the semiconductor wafer. To scale up the power, several VECSELs can be arranged in an array. For thermal reasons, many small devices are more suited than few large devices. A small active diameter and therefore small beam waist means a high divergence angle, reducing the brightness of such an array. To lower the divergence angle a micro lens array can be used collimating the radiation of each VECSEL and thus lowering the beam parameter product and increasing the brightness of the VECSEL array. But an additional adjustment step is necessary for this lens array, either on chip-level or on wafer level, if stacked optical transparent wafers are used.